The present invention relates to the field of optical measurement and characterization of a variety of types of particles suspended in a fluid. In particular, methods and apparatus are described for extending conventional boundaries of particle hydrodynamic radius measurement to both higher and lower particle concentration, and further for extending depolarized dynamic light scattering capabilities used for shape assessment.
Dynamic Light Scattering (DLS) is used extensively in research laboratories and elsewhere for the development of new materials and processes, and less commonly is used for process monitoring and control. Commercial applications exist in several industries, including but not limited to: pharmaceuticals (small molecules and protein therapeutics), medical diagnostics (histology, bodily fluids, cataracts), consumer products (personal care, cosmetics, paints, detergents), chemicals, environmental monitoring and remediation (particulate and biological pollutants, oil spill cleanup), advanced materials (powders, coatings, surfactants), and microelectronics (planarization slurries, thin films). Depolarized Dynamic Light Scattering (DDLS) is similar to DLS, but uses polarization techniques to assess deviations from particle sphericity.
DLS relies on the detection of the Doppler shift of coherent radiation scattered from small colloidal particles suspended in a transparent liquid and undergoing Brownian motion, whose behavior depends upon their hydrodynamic radii and/or shape. DLS is commonly used for determining the translational diffusion coefficient of macromolecules such as proteins and polymers, as well as that of larger colloidal particles, typically up to several microns. Because the hydrodynamic radius of a spherical particle may be determined simply from its diffusion coefficient (and the viscosity of the suspending liquid), dynamic light scattering has become the method of choice for characterizing colloidal particles. The phase and frequency of light scattered from many particles is detected as a fluctuating intensity in the far-field as the suspended particles diffuse. DDLS acquires information about the rotational diffusion coefficient of the particles, which depends on the particle size and shape, and may be extracted by suitable mathematical techniques from the fluctuating intensity of the depolarized detected light.
DLS is a preferred measurement technique for the thermally driven diffusion coefficient of particles in suspensions appearing translucent, those with an extinction length from a few millimeters to a few meters. The extinction length, based on Beer's Law, is that distance from the entrance into a medium to where the propagating beam intensity has declined to e−1 of its incident intensity. However, several important application areas are outside these traditional limits of DLS. For example, there is great interest in measurement of suspensions approaching opacity, as in process monitoring of high concentration slurries. As the concentration of particles in a suspension increases, the opportunity for scattered light to scatter from more than one particle before arriving at a detector also increases. The resultant multiple scattering statistically yields a higher frequency signal and, consequently, a falsely low measurement of radius. At the opposite extreme, there is also strong interest in the measurement of highly transparent suspensions, such as for environmental monitoring and for characterization of dilute suspensions of nanoscale particles and proteins that scatter very little light. To date, technical solutions to measure accurately in both these regimes remain unsatisfactory.
DDLS has not yet become a popular method for measurement because depolarized signals are typically weak and therefore often obscured by interfering signals such as stray light, optical imperfections, or other system noise sources. In addition, depolarized time correlation functions often decay many times more rapidly than those of DLS, especially for small particles. Increased frequency and reduced signal both present challenges for typical detectors and their following electronics. Typical photodetectors optimized for weak signals, e.g., photomultiplier tubes, also suffer from dead time, after-pulsing and noise problems that arise from detecting a small number of photons per correlation time of the relaxation process.